Slavemaker ants raid other ant colonies to bring back pupae, who then grow up to be the labor force. Now it turns out these social parasites show remarkable strategy and intelligence in choosing their future slaves.

Nature is often very brutal, but even so slavemaker ants are really something else. Like other ant colonies, they have a queen that produces all the offspring, but none of these ants are the worker ants needed for foraging and caring for young. Slavemaker colonies only produce worker scouts, who are then charged with finding other ant colonies to invade in search of unborn young, or pupae.

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Once they've raided the colony, the ants bring the pupae back home. Here, the odors of the slavemaker colony are imprinted on the pupae, which makes them loyal (for lack of a better word) to their new colony. These stolen pupae grow up to be the worker ants of the slavemaker colony, sustaining the queen and scouts until it's time to invade another colony. This process is known as social parasitism.

Researchers at Munich's Ludwig Maximilian University wanted to better understand how slavemaker ants go about choosing their "hosts." They hypothesized that the ants would, when given a choice of colonies, choose the weakest or least defended ones because they would be the easiest to invade.

But they discovered the ants did the complete opposite, going after the most strongly defended colonies. This was shocking behavior, as researcher Sebastian Pohl explains:

"At first, we were quite surprised, as we expected that attacking slavemaker colonies prefer host colonies that provide a better benefit to risk ratio. We hence had to look at the slavemakers' decision in more detail and had to consider more aspects of the complete raiding behaviour."

The researchers considered the situation ants would find themselves in just before a raid. A typical slavemaker colony is very small, with the single queen, two to five scouts, and perhaps 30 to 60 slaves. That means scouts are at a premium, and the colony can't risk losing even a single scout in a failed raid.

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Of course, that does mean slavemakers have to target the absolute strongest colonies, which the researchers believe they associate with lots of pupae. In that case, it's better to attack one highly defended colony where they know there will be lots of pupae, rather than ten lightly defended colonies where there might not be enough to sustain them.

It's some wonderfully counterintuitive thinking on the part of the slavemaker ants - after all, the scientists guessed they would be doing the exact opposite. It's a brutal practice, but you've got to admit the slavemaker ants seem to know what they're doing.